VBox aborts virtual machine instance with port forwarding

I'm using VirtualBox 1.5.0 (binary version) on ArchLinux. I've got a fresh Debian Etch installation called "Debian Etch Webdev". Networksettings for the VM are default NAT settings. After enabling portforwarding as described in the documentation, the VM aborts when trying to setup networking in the VM. VirtualBox saves the attached log file into my home.